Field of the Invention
The present invention relates to a zoom lens, which is suitable for, for example, an image pickup optical system used in an image pickup apparatus such as a digital still camera, a video camera, a monitoring camera, a broadcasting camera, a silver-halide film camera, or the like.
Description of the Related Art
In recent years, image pickup apparatus using solid-state image pickup elements or silver-halide films have been increased in functionality while the entire apparatus have been reduced in size. As image pickup optical systems to be used in those image pickup apparatus, a small zoom lens having a high zoom ratio, a wide angle of view, and high optical performance over an entire zoom range even in the periphery of a screen has been sought for. As a zoom lens that is small as the entire system and has a high zoom ratio, there has been known a positive lead type zoom lens including, in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a rear lens group including at least one lens unit arranged subsequently to the third lens unit.
In a zoom lens, aberration that has occurred in a first lens unit is enlarged by a second lens unit and subsequent lens units. Thus, when a higher zoom ratio is realized, it is important to suppress aberration occurring in the first lens unit as much as possible. Among various aberrations occurring in the first lens unit, aberration that occurs in a large amount is the secondary spectrum of axial chromatic aberration at a telephoto end.
Hitherto, in order to suppress the secondary spectrum of axial chromatic aberration at a telephoto end, a low dispersion material having high relative partial dispersion has been used for a positive lens in a first lens unit. Moreover, in order to correct the above-mentioned secondary spectrum of the axial chromatic aberration that has occurred in the first lens unit, a material having high relative partial dispersion has been used for a positive lens in a third lens unit having a positive refractive power.
In Japanese Patent Application Laid-Open No. 2010-91788, there is disclosed a zoom lens having a high zoom ratio in which anomalous dispersion materials are used as materials of positive lenses in a first lens unit and a third lens unit, to thereby satisfactorily correct the secondary spectrum of chromatic aberration. In Japanese Patent Application Laid-Open No. 2010-32700, there is disclosed a zoom lens in which a third lens unit includes a lens formed of a material having an anomalous dispersion characteristic.
When a material having a strong anomalous dispersion characteristic is used to suppress the secondary spectrum of axial chromatic aberration at a telephoto end as in the zoom lenses disclosed in Japanese Patent Application Laid-Open No. 2010-91788 and Japanese Patent Application Laid-Open No. 2010-32700, lateral chromatic aberration is increased at a wide angle end. In order to satisfactorily correct axial chromatic aberration and lateral chromatic aberration over an entire zoom range while realizing a higher magnification, it is important to appropriately set a power arrangement of each of lens units and materials of lenses in each of the lens units.